This project is designed to determine how the lactotroph cell perceives dopamine as both a prolactin-inhibiting hormone and a prolactin-releasing hormone. This will be accomplished by pursuing 5 specific aims. In the first aim, we will determine the role of dose, temperature and pattern of administration on the stimulatory or inhibitory effects of dopamine on prolactin secretion in vitro. In the second specific aim we will determine with the reverse hemolytic plaque reaction if the cell types which are responding in this dual manner to dopamine are exclusively lactotrophs and/or somatomammotrophs. In the third aim we will characterize the nature of the intracellular mechanisms responsible for both the stimulatory and inhibitory response to dopamine. In the fourth aim we will characterize the interaction of known stimulators of prolactin with the inhibitory and stimulatory response to dopamine. In the fifth aim we will determine how the rat uses the stimulatory properties of dopamine in controlling prolactin secretion in various physiological situations. These experiments will use techniques already established in our laboratories to answer physiological questions of importance to all mammals known to use dopamine as a neurohormone which controls prolactin secretion. In addition, these studies may suggest strategies through which clinicians might wish to control prolactin secretion.